Jonathan Lee Yacketta, 31, has been scheming Pinellas County residents for the past several years, authorities said.

"There's others in the area doing this," said Pinellas County sheriff's Detective Stephen Bingham. "But he's one of the biggest."

Yacketta pleaded guilty to 16 charges, including exploitation of the elderly, grand theft and soliciting without a permit.

Prosecutors said he duped elderly men and women, many suffering physical and mental illnesses, by charging them thousands of dollars for water filtration systems, warranties and service contracts. He found his latest victim, an 88-year-old woman with a heart condition, last month while he was out on bail for several other fraud charges. His bail was revoked.

A plea agreement caps Yacketta's prison sentence at 10 years. Pinellas-Pasco Circuit Judge Philip Federico will sentence him Oct. 21. Between now and then, Yacketta vowed to find money toward restitution in hopes of reducing his sentence length. But he'll have to do that from the Pinellas County Jail. "I apologize," he said in court Friday, after changing his mind several times about signing the plea deal. "I'm going to pay all of it."

The scheme usually works like this: Elderly people are offered water filtration systems that normally cost a few hundred dollars. Salesmen, like Yacketta, would make false claims about the system's value and convince victims they needed his services to maintain the devices. "What Jonathan Yacketta did is triple and quadruple charge these victims," Bingham said. "He was making tons of money each month."

"It's terrible," said Jim Vann, 57, of St. Petersburg, whose mother is one of the victims. "He did this to so many people. It's just a shame." Vann and his brother discovered their 88-year-old mother, who suffers from dementia, had written more than $30,000 in checks to Yacketta since 2005.